Orthodontic treatment often involves attaching an appliance to the tooth. Forces applied to the appliance are then transferred to, and thus move, the tooth. As such, orthodontic appliances represent a principal component of corrective orthodontic treatment devoted to improving a patient's dentition. Orthodontic appliances may include brackets, archwires, or other devices.
Using the orthodontic bracket as an example, an orthodontist may affix orthodontic brackets to the patient's teeth with an adhesive and engage an archwire into a slot of each bracket. The archwire exerts flexural and/or torsional stresses on the orthodontic brackets to create restorative forces, including rotation, tipping, extrusion, intrusion, translation, and/or torque forces, tending to bring the teeth toward a desired position. Traditional ligatures, such as small elastomeric O-rings or fine metal wires, may be employed to retain the archwire within each bracket slot. Due to difficulties encountered in applying an individual ligature to each bracket, self-ligating orthodontic brackets have been developed that eliminate the need for ligatures by relying on a movable portion or member, such as a latch, clip, or slide, for retaining the archwire within the bracket slot.
Various materials have dominated the orthodontic market because of their characteristic combination of strength, toughness, aesthetics, biological/corrosion resistance, and manufacturability. For example, archwires used early in orthodontic treatment may be made of shape memory alloys (SMAs) with superelastic properties that have corrosion resistance. Nitinol is a well-known shape-memory alloy and is an alloy of nickel (Ni) and titanium (Ti). By way of additional example, archwires may be made of stainless steel or a Ti-containing alloy, such as, a titanium molybdenum alloy (TMA). Each of these metals combines some level of strength, toughness, and corrosion resistance. Similarly, orthodontic brackets are ordinarily formed from stainless steel, which is strong, nonabsorbent, weldable, and relatively easy to form and machine. Titanium and titanium alloy brackets are also available as nickel-free alternatives to stainless steels. Though titanium is more corrosion resistant than stainless steel, titanium is more expensive and more difficult to manufacture than stainless steel.
As an alternative to the metallic orthodontic brackets, certain orthodontic brackets incorporate a bracket body of a transparent or translucent non-metallic material, such as a ceramic, that assumes or mimics the color or shade of the underlying tooth. However, as compared to their metallic counterparts, ceramic orthodontic brackets have a comparatively low strength and toughness. Furthermore, ceramic appliances are difficult and costly to manufacture.
Injection molding processes are capable of producing intricately formed parts, such as, metallic or ceramic orthodontic appliances. The parts formed are often referred to as green bodies and include a formed mixture of unsintered powder and binder. The unsintered powder may include metallic or ceramic powder. Powder injection molding (PIM) with either of these powders may be referred to as metal injection molding (MIM) or ceramic injection molding (CIM), respectively. The mixture of unsintered powder and binder is heated to soften the binder and the heated mixture is then injected into a mold. Once injected, the binder cools and hardens so as to hold the particles together in the injection molded form. To form the final product, the binder is removed. The binder-free bodies are then heated to an elevated temperature to sinter the particles together, thus consolidating the powder particles into a sintered body. Subsequent finishing operations may be required to transform the sintered body into the final orthodontic appliance.
To form the orthodontic appliances via MIM or CIM processes, the binder material must be removed during subsequent processing. This process may be referred to as debinding. Typically, the green bodies are therefore subject to a process whereby the binder is removed. Removal of the binder is often achieved by heating the green body in a furnace to an elevated temperature at which the binder decomposes or gasifies. A flow of gases inside the furnace transports the gasified or decomposed binder away from the green body. In addition to furnaces or thermal-based systems, chemical-based and water-based systems are also known methods for removing the binder from the green body. Depending on the size of the green body and the type of binder, as well as other factors, removal of the binder may take a significant period of time, typically in the range of hours, if not longer. Thus, long periods required for binder removal delay subsequent sintering operations and result in an overall lengthening of the manufacturing process. Furthermore, each of these processes may utilize significant amounts of energy, require expensive and/or bulky equipment, and produce a waste stream of gases or other chemicals that are typically the subject of environmental regulation.
Consequently, there is a need for improved systems and methods of manufacturing orthodontic appliances that overcome these and other deficiencies.